Stem cells are defined to be cells which are capable both of self-renewal and differentiation into one or more differentiated cell types. Human embryonic stem cells are a category of stem cells created from human pre-implantation blastocysts. Human embryonic stem cells are pluripotent and may be totipotent, meaning that they can certainly differentiate into many cell types evidenced in an adult human body and may be capable of differentiating into all cell types present in the human body.
Embryonic stem cells (ES cells) have also been derived in a number of animals other than humans. For example, much scientific work has been conducted with murine ES cells. Once a method for the initiation of ES cell cultures for a particular species is worked out, it becomes possible to manipulate the ES cells, and animals which result therefrom, in a variety of ways to learn useful information about the genetics of the animal under study. For example, it has become possible over the past decade to create cultures of murine ES cells in which one or another specific gene is knocked out in each murine stem cell culture. While some techniques that could be worked out in murine ES cell systems were transferable to other species, many were not. For example, the basic techniques which could be used to create murine ES cell cultures did not transfer well to many other animal species. For the development of techniques for the culture and manipulation of human ES cells, the murine cell may therefore not be the best model due to the phylogenic distance between humans and mice. However, in the course of the development of the science of human ES cell cultures and techniques, much of the preliminary work was conducted in non-human primates, such as the rhesus monkey. Other primate ES cell cultures have proven to be a relatively reliable model for systems that could be easily transferred to human cell culture. For an example, murine ES cell cultures require application of leukemia inhibitory factor (LIF) or another agonist of the gp130/STAT3 signaling pathway for maintenance of undifferentiated cell growth, whereas human and rhesus monkey ES cell cultures do not require LIF for undifferentiated cell growth. Prior work on hematopoiesis using rhesus monkey ES cells validates the utility of this system for doing pre-clinical investigations for techniques that can be transferred to human ES cell cultures.
One of the exciting potential uses of stem cells is for human tissue transplantation. It is hoped and expected that techniques can be developed to direct the differentiation of stem cells into specific lineages which can then be transferred into the human body to replace or enhance tissues of the body. In order to do that, first techniques must be developed to direct the differentiation of stem cells into the specific cell lineages desired. Techniques have already been proposed to direct stem cell cultures into lineages of hematopoeic, neural, cardiomyocyte, pancreatic and other lineages. These techniques have proven to be quite different from each other and independent in the sense that a new and different technique is required for each new desired lineage.
Endothelial cells make up a network of interconnected cells in the human body that line blood vessels, lymphatic vessels, and form capillaries. Endothelial cells regulate the flow of nutrient substances and create and respond to diverse biologically active molecules. While it has been demonstrate that human ES cells will differentiate into many progeny cells types, including endothelial cells, it has not been previously possible to create distinct cultures of derivatives of human ES cells directed into an endothelial lineage.